Lesson07 - H O HCN H O Structures That Have Delocalized Electrons A valid Lewis structure for CH3NO2 is shown above You may have realized that the

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Structures That Have Delocalized Electrons A valid Lewis structure for CH 3 NO 2 is shown above. You may have realized that the double bond between oxygen and nitrogen could just as well have been drawn from the top oxygen. In other words, no single Lewis structure accurately represents the distribution of electrons in CH 3 NO 2 . Rather, to better represent the electron configuration, this structure requires two resonance contributors. The two contributors are shown below, along with the resonance hybrid. The two contributors are identical; therefore, we expect each to be equally important to the structure’s “true” electron configuration. Consequently, the two oxygen atoms share the negative charge equally, as shown in the hybrid. Notice that we don’t bother showing lone pairs in the hybrid, since there is no easy way to represent 2.5 lone pairs (showing a half pair as a single dot • is not a good idea, because the • symbol has a different meaning to chemists). This example reinforces the idea that when considering the structure of organic molecules, always remember to draw as many valid Lewis structures as you can generate. H C H N H O O H C H N H O O H 3 C N O O -1/2 -1/2 H C H N H O O resonance contributors resonance hybrid The delocalization of electrons allows us to write alternative Lewis structures in which the formal charges move.
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Curved Arrow Notation Curved arrow notation is a convention to show changes in electron configuration. It is one of the most practically useful skills that you will develop in CHEM 232, and you’ll continue to use it in CHEM 332, biochemistry and beyond. Curved arrows will be used to show changes in electron configuration that accompany each step of a chemical reaction. Here we will use curved arrows to change one resonance contributor into another. The figures below outline the step-by-step process. The process begins (Step 1) by identifying electron pairs associated with electron “rich” regions of the molecule – atoms having a surplus of electron density. For the structure in question, the most obvious electron rich building block is the negatively charged oxygen. There are three electron lone pairs on this oxygen. One has been circled but any one of the three could have been selected. We will call this pair the electron source (Step 2). In step 3, a curved arrow is used to depict the process of transferring the electron source to a position located between oxygen and nitrogen to which it is bonded. Curved arrows are highly revered in organic chemistry. When you draw arrows, be careful to precisely position the tail and head. The tail of the arrow is located exactly on the electron pair undergoing the
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This note was uploaded on 01/25/2012 for the course CHEM 232 taught by Professor Miller during the Spring '08 term at University of Illinois, Urbana Champaign.

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Lesson07 - H O HCN H O Structures That Have Delocalized Electrons A valid Lewis structure for CH3NO2 is shown above You may have realized that the

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